Semiconductor devices have been more and more miniaturized. The semiconductor devices are mass-produced by repeatedly applying a photolithography process to form different layers of material. In a photolithography process, a photomask having a pattern is irradiated with light to transfer the pattern onto a photosensitive coating on a semiconductor substrate (hereinafter, referred to as “wafer”) via a reduction optical system. To improve photolithography resolution so smaller features can be patterned, the exposure wavelength of optical lithography have been shortened. In recent years, photolithography that uses extreme ultraviolet (EUV) light (wavelength=13.5 nm) having a much shorter wavelength has been in development. However, at short wavelengths for EUV lithography, the electromagnetic radiation is absorbed by most materials, including glass used for conventional lenses and masks. Therefore a completely different tool is necessary for performing EUV lithography compared to conventional photolithography.
Some EUV lithography tools employ reflection type photomasks. A reflective EUV lithography mask (hereinafter, referred to as EUV mask) is composed of a reflective multilayer (ML) coating of alternatively stacked films of, for example, molybdenum (Mo) films and silicon (Si) films. The films are deposited on low thermal expansion material (LTEM) such as a quartz substrate or low-thermal-expansion glass substrate. An absorber pattern is formed on the reflective ML coating. The multilayer film is a dielectric mirror that reflects light through constructive interference.
Because of the extremely short exposure wavelength of 13.5 nm, a small perturbation in the surface of the dielectric mirror causes a phase defect in the reflection that can blur the pattern transferred onto a wafer. Some phase defects are caused by pits created during substrate polishing. Some phase defects are caused by particles on a substrate that cannot be removed by cleaning. Improvements in structures of and methods of forming EUV masks that reflect with little or no phase defects continue to be sought.